Method for operating an internal combustion engine with an engine comprising a number of cylinders and an injection system with high-pressure components, in particular an injection system comprising a common rail with a number of injectors associated with the cylinders, in particular wherein a single reservoir that is embodied for holding fuel from the common rail for an injector is associated with the injector.
The concept of an injector with a single reservoir in the context of a common-rail injection system has been proved, such as for example as is described in DE 199 35 519 C2 by way of example. The single reservoir is supplied with fuel under pressure via a fuel feed channel from the pressure connector and has a direct fluid connection to the high-pressure channel for the fuel under high pressure in the common rail. The volume of the single reservoir is large compared with the volume of the high-pressure channel and the nozzle pre-chamber in the injector. Because of the arrangement of the injector—possibly decoupled from the common rail by means of a choke element—there is sufficient space within the housing of the fuel injector in the single reservoir to provide fuel for at least one complete injection quantity for a working cycle of a cylinder, but in any case for a partial injection during the working cycle.
DE 10 2009 002 793 B4 discloses a single reservoir or a high-pressure component such as a common rail with a pressure measuring device embodied in the form of a strain sensor, wherein the strain sensor is embodied in the form of a strain gauge and is disposed on the outside of a wall of the single reservoir, and a hydraulic resistance is disposed immediately upstream or downstream of the single reservoir for integration within the high-pressure feed.
When starting the engine, on the one hand it must be ensured that the high pressure does not exceed a maximum value of, for example, 600 bar specified by the pump manufacturer, because otherwise the pump can be damaged because of the excessive counter-pressure. On the other hand, the high pressure should be as high as possible when starting the engine in order to ensure good acceleration behavior and low emissions.
The actuation of the suction choke when starting the engine according to the prior art is described in the patent specification DE 101 56 637 C1. In this case, the suction choke is energized with a constant energization value, preferably 0 A, with the engine off or with the engine running until reaching a high pressure threshold value of, for example, 800 bar. On reaching the threshold value, the high pressure control is activated, whereby the suction choke is energized so that the high pressure is controlled to the setpoint high pressure. Said method is particularly advantageous for common-rail systems with a large system leak. With systems of this type, the rail pressure, i.e. the fuel pressure in the common rail, decreases rapidly to a low value after the engine is stopped, for example to 0 bar. If the suction choke is initially not energized in this case after starting the engine, then a maximum rise in the high pressure is achieved up to a specifiable high-pressure threshold value. This enables a rapid and reliable engine start, because on the one hand injections in common-rail systems are only possible if the opening pressure of the injection nozzles is achieved. The magnitude of said opening pressure is usually 350-400 bar. On the other hand, the engine can be accelerated faster at higher high pressures, because the fuel is combusted better in this case, whereby higher efficiency results.
While this is correct in principle, nevertheless the following problem has proved to be relevant: with new common-rail systems, actuation of the suction choke according to the prior art is less advantageous, because said systems only have a slight system leak. The result of this is that the high pressure is not decreased when stopping the engine and therefore remains at values that prevail at the point in time of stopping. Because the engine is operated at high pressures of 600-2200 bar, before starting the engine as a rule a high pressure prevails that could damage the high-pressure pump of the injection system.
It is therefore desirable to set the pressure prevailing within the injection system at the point in time of starting the engine within a predetermined range of values that is low enough in order to not damage the high-pressure pump of the injection system, and at the same time is high enough in order to have good acceleration behavior and advantageous emission behavior.
In order to satisfy the aforementioned requirements in an improved manner, a method must be developed that sets the pressure prevailing within the injection system at the point in time of starting the engine consistent with a predetermined range of values.